Method, system and equipment for retransmission in communications systems

ABSTRACT

By adaptively relocating a transmitting point between a base station and a base station controller, e.g., an RNC, optimal ARQ performance is achieved. The adaptation may be based on one or more of many factors such as the distance between a network element and a user equipment. As one example, for a mobile radio near a base station, the transmitting point where the ARQ protocol terminated in the radio network is located in that base station so that ARQ packet retransmissions are performed quickly. For a mobile radio in or about to be in soft handover, the transmitting point where the ARQ protocol terminated in the radio network is located in a base station controller coupled to the base stations involved in (or soon will be involved in) soft handover so that robust ARQ packet retransmissions are provided.

This application is the US national phase of international applicationPCT/SE02/00231 filed 08 Feb. 2002, which designated the US.

TECHNICAL

The present invention relates to retransmissions in a communicationssystem, and more especially it relates to a cellular mobile radiosystem, particularly to a Universal Mobile Telecommunications System,UMTS.

BACKGROUND

Retransmission of data to or from a mobile station, MS, or userequipment, UE, is previously known. It is also known to use a radio linkcontrol layer of a UMTS protocol structure in an acknowledged mode fordedicated channels.

In acknowledged mode, retransmissions are undertaken in case of detectedtransmission errors not recovered by forward error control. This is alsocalled automatic repeat request, ARQ. With ARQ, retransmissions can beundertaken unless a transmitted message is (positively) acknowledged orif it is negatively acknowledged. Generally there are time limits forthe respective positive and negative acknowledgements to be considered.

Within this patent application, a radio network controller, RNC, isunderstood as a network element including a radio resource controller.Node B is a logical node responsible for radio transmission/reception inone or more cells to/from User Equipment. A base station, BS, is aphysical entity representing Node B.

Radio link control, RLC, is used within radio communications systemslike General Packet Radio Services, GPRS, and UMTS.

International Patent Application WO0105121 describes a technique forproviding a secure link in a mobile communication system includingmechanisms for hard handover of a link in acknowledged mode. Data istunneled.

A channel dedicated to a specific UE is referred to as a DedicatedChannel, DCH. A channel that is not a dedicated channel is called acommon or shared channel.

UK patent application GB no. 0027148.6, describes channel switchingbetween dedicated and common channels.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.301v3.6.0, France, September 2000, specifies in chapter 5 Radio InterfaceProtocol Architecture of a UMTS system. There are three protocol layers:

-   -   physical layer, layer 1 or L1,    -   data link layer, layer 2 or L2, and    -   network layer, layer 3 or L3.

Layer 2, L2, and layer 3, L3 are divided into Control and User Planes.Layer 2 consists of two sub-layers, RLC and MAC, for the Control Planeand 4 sub-layers, BMC, PDCP, RLC and MAC, for the User Plane. Theacronyms BMC, PDCP, RLC and MAC denote Broadcast/Multicast Control,Packet Data Convergence Protocol, Radio Link Control and Medium AccessControl respectively.

FIG. 1 illustrates a simplified UMTS layers 1 and 2 protocol structurefor the so called Uu Stratum, UuS, or Radio Stratum, between a userequipment UE and a Universal Terrestrial Radio Access Network, UTRAN.

Radio Access Bearers, RABs, make available radio resources (andservices) to user applications. For each mobile station there may be oneor several RABs. Data flows (in the form of segments) from the RABs arepassed to respective Radio Link Control, RLC, entities which amongstother tasks buffer the received data segments. There is one RLC entityfor each RAB. In the RLC layer, RABs are mapped onto respective logicalchannels. A Medium Access Control, MAC, entity receives data transmittedin the logical channels and further maps logical channels onto a set oftransport channels. One transport channel is Downlink Shared Channel,DSCH.

Transport channels are finally mapped to a single physical transportchannel which has a total bandwidth allocated to it by the network. Infrequency division duplex mode, a physical channel is defined by code,frequency and, in the uplink, relative phase (I/Q). In time divisionduplex mode a physical channel is defined by code, frequency, andtime-slot. The DSCH, e.g., is mapped onto one or several physicalchannels such that a specified part of the downlink resources isemployed.

PDCP provides mapping between Network PDUs (Protocol Data Units) of anetwork protocol, e.g. the Internet protocol, to an RLC entity. PDCPcompresses and decompresses redundant Network PDU control information(header compression and decompression).

For transmissions on point-to-multipoint logical channels, BMC stores atUTRAN side Broadcast messages received from an RNC, calculates therequired transmission rate and requests for the appropriate channelresources. It receives scheduling information from the RNC, andgenerates schedule messages. For transmission the messages are mapped ona point-to-multipoint logical channel. At the UE side, BMC evaluates theschedule messages and deliver Broadcast Messages to upper layer in theUE.

3G TS 25.301 also describes protocol termination, i.e. in which node ofthe UTRAN the radio interface protocols are terminated, or equivalently,where within UTRAN the respective protocol services are accessible.Section 5.6.5 describes protocol termination for DSCH. The RLC protocolfor DSCH is terminated in Serving Radio Network Controller, SRNC, forboth the control and user planes.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.322v3.5.0, France, December 2000, specifies the RLC protocol. The RLC layerprovides three services to the higher layers:

-   -   transparent data transfer service,    -   unacknowledged data transfer service, and    -   acknowledged data transfer Service

Subsections 4.2.1.1 and 4.2.1.2 describe transparent mode entities andunacknowledged mode entities. Basically, RLC differences of the twomodes reside in management of packet overhead. In transparent mode nooverhead is added or removed by RLC. In subsection 4.2.1.3 anacknowledged mode entity, AM-entity, is described (see FIG. 4.4 of the3GPP Technical Specification). In acknowledged mode automatic repeatrequest, ARQ, is used. The RLC sub-layer provides ARQ functionalityclosely coupled with the radio transmission technique used. The threemodes

-   -   Transparent Mode, TM,    -   Unacknowledged Mode, UM, and    -   Acknowledged Mode, AM        are hereinafter collectively referred to as RLC modes.

None of the cited documents above discloses a dynamic RLC configurationand termination point.

SUMMARY

In accordance with 3G TS 25.301, no macrodiversity is applied for DSCH,i.e., a specific DSCH is transmitted in a single cell only. As describedabove, the RLC protocol, and correspondingly the ARQ, is terminated inthe serving RNC (SRNC). However, when the DSCH is transmitted in only asingle cell at a time, the retransmission delay would reduceconsiderably if retransmissions were terminated in the BS, as the roundtrip delay would be decreased.

When a UE is distant to a base station, retransmission requests arelikely to be transmitted to one or more base stations that did nottransmit data at first instance. Retransmission will then involvetransmissions between the two or more BSes and involve an RNC,nullifying the advantage of a termination point located in the BS, andpotentially increasing retransmission delay.

Further, when a user equipment, like UE 1 in FIG. 2, is not involved insoft handover, it is advantageous in terms of time delay to haveretransmissions terminated in BS 1/Node B 1, whereas when it is involvedin soft handover it is advantageous to have the termination pointlocated at an RNC.

Consequently, an object is to achieve a method and system of fastretransmissions to a UE near a BS and allowing for robustretransmissions using soft handover when necessary.

It is also an object to present an adaptive relocation of transmittingpoint.

These objects are met by dynamically switching between RLCconfigurations depending on radio conditions and UE locations. The RLCconfiguration and the location of transmitting point are adapted to UElocation, number of transmitters and PDU size. Relocation is preferablyachieved by means of RLC tunneling.

Preferred embodiments, by way of examples, are described with referenceto the accompanying drawings below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays a layered protocol structure in a radio communicationssystem.

FIG. 2 schematically illustrates a user equipment communicating with oneor two base stations.

FIG. 3 shows communication with ARQ terminated in Node B between a UEand a base station involved in a connection between RNC and the UE.

FIG. 4 shows communication with ARQ terminated in RNC between a UE andtwo base stations involved in a connection between RNC and the UE.

FIG. 5 schematically illustrates two protocol layers from a multilayerprotocol.

FIG. 6 illustrates the principle of RLC relocation.

DETAILED DESCRIPTION

With reference to FIG. 3, Node B 1 and Node B 2 are logical nodesresponsible for radio transmission/reception in one or more cellsto/from the User Equipment UE. BS 1 and BS 2 are physical entitiesrepresenting Node B 1 and Node B 2 respectively. Node B 1 and Node B 2terminate the air interface, called Uu interface within UMTS, between UEand respective Node B towards the radio network controller RNC. In UMTSthe interface between a Node B and an RNC is called Iub interface.

In an exemplary situation UE communicates over a radio link associatedwith BS 1. Packet switched data is transmitted in protocol data units,PDUs, in both directions. If a protocol data unit PDU is received inerror and the error is not recovered by forward error correction, thePDU is retransmitted.

In accordance with 3G TS 25.301, no macrodiversity is applied for DSCH,i.e., a specific DSCH is transmitted in a single cell only. As describedabove the RLC protocol, and correspondingly the ARQ, is terminated inthe RNC. However, when the DSCH is transmitted in only a single cell ata time, the retransmission delay is reduced considerably by terminatingretransmissions in BS 1 or Node B 1 in place of the RNC as the roundtrip delay is thereby decreased.

An ARQ-machine is a physical entity from which retransmissions areinitiated. In FIG. 3 there is an ARQ-machine ARQ in each of Node B 1 andUE.

With reference to FIG. 2, user equipment UE 1 being close to a basestation BS 1 will in general not be involved in soft handover. A userequipment more distant to BS 1, like user equipment UE 2, is likely tocommunicate over radio links associated with more than one BS, BS 1 andBS 2. In this figure the BSes are indicated to operateomnidirectionally. However, the technology is not limited toomnidirectional base stations. It can readily be used irrespective ofwhether the base stations use directional or omnidirectional antennaradiation patterns.

Excessive retransmissions reduce throughput and system performance. Softhandover can reduce the amount of transmission errors not recovered.

In FIG. 4, user equipment UE is involved in soft handover for at leastone link direction (up or down). A macrodiversity or soft handovercombiner for the uplink direction Comb is located at the RNC, see FIG.4. When soft handover is used also in downlink direction a correspondingcombiner in user equipment UE is utilized. Depending on the outcome ofthe macrodiversity combining, there may be no need for retransmission.Further, when a UE is distant to a base station, retransmission requestsare likely to be transmitted to one or more base stations that did nottransmit data at first instance. Retransmission will then involvetransmissions between two or more base stations/Nodes B and involve anRNC, nullifying the advantage of a termination point located in Node B,and potentially increasing retransmission delay. In FIG. 4 theARQ-machine ARQ is located in RNC and UE respectively.

Consequently, when user equipment UE is not involved in soft handover,it is advantageous in terms of time delay to have retransmissionsterminated in a Node B, Node B 1, see FIG. 3. When it is involved insoft handover with radio links associated with two or more Nodes B, NodeB 1 and Node B 2, it is advantageous to have the termination pointlocated at radio network controller RNC, see FIG. 4.

Thus, an ARQ machine, not being fixed to one geographical location, isintroduced which can be dynamically relocated between a Node B and anRNC as need be to achieve a sufficiently small retransmission delay.Four example alternatives of relocation of an ARQ-machine areconsidered:

-   -   1. Protocol State Transfer,    -   2. Multiple ARQ Protocols,    -   3. Service Data Unit Transfer, and    -   4. RLC Tunneling.

A network layer PDU or L3 PDU can comprise several RLC PDUs, asillustrated in FIG. 5. RLC PDUs are reassembled into service data units,SDU, prior to delivery to higher layer PDU. The L3 protocol can be,e.g., the Internet Protocol, IP. Upon reception from L3, SDUs aresegmented into RLC PDUs.

Protocol State Transfer moves/transfers the whole protocol state,including state variables and buffers to the new network node.

With Multiple ARQ Protocols, data is secured by having two or morelevels of ARQ protocols. One protocol level is run between UE and NodeB, another protocol level is run between UE and RNC. Upon relocation, noparticular measures need to be undertaken for PDUs in the old ARQmachine, as a potential loss of data is recovered by higher level ARQprotocols.

In Service Data Unit transfer, SDUs are buffered until all RLC PDUscarrying an SDU are successfully transmitted. Upon relocation, allstored (complete) SDUs are moved from the old ARQ-machine to the new ARQmachine. The SDUs are segmented into RLC PDUs and transmitted at the newARQ-machine.

Finally, using RLC tunneling for relocation of an ARQ-machine there willbe two RLC protocols considered: the old/existing RLC protocol and a newRLC protocol at the new location. One or more RLC PDUs buffered but notyet successfully transmitted to the destination from the old RLCprotocol are tunneled through the new RLC protocol. The old RLC protocoldoes not perform the ARQ function of the tunneled RLC PDUs. In reversedirection, the old RLC protocol assembles old RLC PDUs provided by thenew RLC protocol until a SDU or L3 PDU, only partially completed at thetime of relocation, is completed. Subsequent SDUs or L3 PDUs will beassembled at the new RLC protocol. In UMTS the ARQ protocols are RLCprotocols. This technology also applies if other than the RLC protocolis used for ARQ.

RLC tunneling also enables RLC reconfiguration in the new retransmissionpoint. This is important as performance can be improved in the newlocation by a change of e.g. PDU size.

In one example embodiment, two or more of the basic alternatives forrelocation are implemented. However, only one alternative is on at atime the other alternatives being switched off.

FIG. 6 illustrates how an ARQ-machine is dynamically relocated. When aUE is close to base station BS 1, the ARQ machine of the UTRAN-side islocated in BS 1. As the UE approaches BS 2 a link associated with BS 2will be established for soft handover. The UTRAN-side ARQ-machine willthen be relocated from ARQ I in Node B 1 to ARQ II in RNC. If the UEmoves further towards base station BS 2 the link associated with BS 1will be released and the ARQ machine relocated from ARQ II in RNC to ARQIII in Node B 1. If only hard handover is used, the ARQ machine ispreferably located in the base stations. Then, relocation of theARQ-machine is from ARQ I in Node B 1 to ARQ III in Node B 2. In FIG. 6the UE-side ARQ-machine is never relocated. However, due toreconfigurations of the ARQ-machine at UTRAN-side, it can bereconfigured accordingly.

FIG. 7 shows two base stations BS 1 and BS 2 and an RNC. Initially onlyBS 1 is communicating with the user equipment UE. The base stationscomprise means 1 for reconfiguring a link layer protocol and means 2 fordetermining one or more communication parameters, such as number ofactive links, propagation path loss, signal to noise ratio, signal tointerference ratio, propagation time. The one or more parameters aredetermined in relation to user equipment UE, involving means 8-10 fortransmission of transmitted and received signal strength, signal timingand interference level respectively used in the process of determiningthe communication parameters. BS 1 and BS 2 also comprise means 3 forchanging PDU size. As PDUs are communicated to and from user equipmentUE it has corresponding means 11. Means 5 and 6 of RNC correspond tomeans 1 and 2 of the base stations since both a base station and an RNCare legitimate RLC termination points. Means 4 of BS 1 and BS 2 andmeans 7 of RNC represent means for transferring a protocol terminationpoint by RLC tunneling, Service Data Unit Transfer, Multiple ARQprotocols or Protocol State Transfer.

A person skilled in the art readily understands that the receiver andtransmitter properties of a BS or a UE are general in nature. The use ofconcepts such as BS, UE or RNC within this patent application is notintended to limit the technology to devices associated with theseacronyms. It concerns all devices operating correspondingly, oradaptable thereto by a person skilled in the art. Explicit non-exclusiveexamples include mobile stations without a subscriber identity module,SIM, as well as user equipment including one or more SIMs. Further,protocols and layers are referred to in close relation with UMTSterminology. However, this does not exclude applicability of thetechnology in other systems such as GPRS or with other protocols andlayers of similar functionality.

The invention is not intended to be limited only to the exampleembodiments described in detail above. Changes and modifications may bemade without departing from the invention. All modifications within thescope of the following claims are covered.

1. A method of dynamically switching a termination point of an ARQ linklayer protocol in a communications system, the method comprising:determining one or more parameters related to a user equipment,dynamically reconfiguring an ARQ link layer protocol residing in theradio network element in a communications system for a communicationwith the user equipment, connecting a network ARQ protocol terminationpoint and a user equipment for the communication with the userequipment, and dynamically switching the network ARQ protocoltermination point for the communication with the user equipment duringthe connection from a first network element at a first hierarchicallevel of the radio network to a second different network element at asecond different hierarchical level of the radio network depending onthe one or more parameters related to the user equipment, wherein thedetermined one or more parameters includes a number of active linkssimultaneously involved in a specific communication service for the userequipment.
 2. The method according to claim 1, wherein one of the firstand second network elements is responsible for radiotransmission/reception.
 3. The method according to claim 1, wherein oneof the one or more parameters is the geographical distance between thefirst network element and the user equipment.
 4. The method according toclaim 1, wherein one of the one or more parameters is the electricaldistance between the first network element and the user equipment interms of at least one of signal propagation path loss, signal to noiseratio, signal to interference ratio, and signal propagation time.
 5. Themethod according to claim 1, wherein the link layer is reconfigured asthe termination point is switched.
 6. The method according to claim 1,wherein the link layer is layer 2 radio link control layer.
 7. Themethod according to claim 1, wherein the location of the ARQ protocoltermination point is changed by any one of: RLC tunneling, Service DataUnit Transfer, Multiple ARQ Protocols, and Protocol State Transfer. 8.The method according to claim 1, wherein the link layer is reconfiguredby changing at least one of: size of packet data units or service dataunits, ARQ discipline, such as no ARQ, Stop and wait ARQ, Go-Back-N ARQ,Selective Repeat ARQ and ARQ using positive or negativeacknowledgements, RLC mode, acknowledgement timing, tunneling state,tunneling destination.
 9. The method according to claim 1, wherein thecommunications system is a cellular mobile radio system.
 10. A radionetwork element comprising: means for determining one or more parametersrelated to a user equipment, means for dynamically reconfiguring an ARQlink layer protocol residing in the radio network element in acommunications system for a communication with the user equipment, meansfor connecting a network ARQ protocol termination point and a userequipment for the communication with the user equipment, and means fordynamically switching the network ARQ protocol termination point for thecommunication with the user equipment during the connection from a firstnetwork element at a first hierarchical level of the radio network to asecond different network element at a second different hierarchicallevel of the radio network depending on the one or more parametersrelated to the user equipments, means for determining the one or moreparameters including a number of active radio links simultaneouslyinvolved in serving the communication service with the user equipment.11. The network element according to claim 10, wherein one of the one ormore parameters is the geographical distance between the first networkelement and the user equipment.
 12. The network element according toclaim 10, wherein one of the one or more parameters is the electricaldistance between the first network element and the user equipment interms of at least one of: signal propagation path loss, signal to noiseratio, signal to interference ratio, and signal propagation time. 13.The network element according to claim 10, further comprising: means forreconfiguration of the link layer upon switching of the network protocoltermination point.
 14. The network element according to claim 10,wherein the link layer is a layer 2 radio link control layer.
 15. Thenetwork element according to claim 10, further comprising means fortransferring a protocol termination point by RLC tunneling, Service DataUnit Transfer, using Multiple ARQ Protocols or Protocol State Transfer.16. The radio network element according to claim 10, wherein the firstradio network element is a base station and the second radio networkelement is a radio network controller coupled to multiple base stations.17. The radio network element according to claim 10, wherein the firstradio network element is a radio network controller coupled to multiplebase stations and the second radio network element is a base station.18. User equipment comprising: an ARQ protocol entity residing in theuser equipment for establishing a link layer protocol connection for acommunication between the user equipment and an ARQ protocol terminationpoint located in a radio base station; and a controller, based on one ormore parameters associated with the user equipment or the ARQ protocoltermination point, for reconfiguring, during the communication, the linklayer protocol connection with the ARQ protocol termination point forthe communication when the ARQ protocol termination point is relocatedduring the communication from the radio base station to a radio networkcontroller associated with the radio base station, wherein the one ormore parameters includes a number of active links simultaneouslyinvolved in a specific communication service for the user equipment. 19.The user equipment according to claim 18, wherein the one or moreparameters includes one or more of: geographical distance between thefirst network element and the user equipment, and electrical distancebetween the first network element and the user equipment in terms of atleast one of: signal propagation path loss, signal to noise ratio,signal to interference ratio, and signal propagation time.
 20. Themethod in claim 1, further comprising: initially locating the ARQprotocol termination point for the connection in the radio base station,and relocating the ARQ protocol termination point in the base stationcontroller when the connection is in soft handover.
 21. The method inclaim 20, further comprising: switching the ARQ protocol terminationpoint from the base station controller to a third network elementcorresponding to another radio base station.
 22. A radio access networkcomprising: a radio base station for establishing a communication with auser equipment and having an ARQ protocol termination point forestablishing a link layer protocol connection with an ARQ protocolentity located in the user equipment, and a radio network controller forcontrolling multiple radio base stations and, based on one or moreparameters associated with the user equipment or the ARQ protocoltermination point, for reconfiguring the link layer protocol connectionso that the ARQ protocol termination point for the communication ismoved from the radio base station to the radio network controller,wherein the one or more parameters includes a number of active linkssimultaneously involved in a specific communication service for the userequipment.
 23. The radio access network according to claim 22, whereinthe one or more parameters includes one or more of: a geographicaldistance between the first network element and the user equipment, andan electrical distance between the first network element and the userequipment in terms of at least one of: signal propagation path loss,signal to noise ratio, signal to interference ratio, and signalpropagation time.